The present invention relates generally to speakerphones, and more particularly to an improved duplex speakerphone that can advantageously be utilized in both radio and landline telephone systems.
In both radio and landline telephone systems, a user typically communicates by means of a handset that includes a speaker at one end which is placed close to the user's ear and a microphone at the other end which is held close to the user's mouth. In operation one hand of the user must be used to hold the telephone handset in its proper orientation thereby leaving the user's only free hand to accomplish tasks, such as driving a vehicle. In order to provide a greater degree of freedom for the user, speakerphones have been developed for use in landline telephone systems.
A conventional speakerphone is typically coupled to the telephone line and, when enabled, allows the user to move about freely while communicating with another telephone user. Such speakerphones generally either receive or transmit at a given instant, i.e. simplex, and a comparison of the volume of the two parties is made. This comparison selects the party speaking the loudest and determines the mode-receive or transmit. If this selection did not occur, a feedback, or echo, may disrupt the communications and, if severe enough, cause an oscillation in the telephone circuit known as "singing".
One type of echo commonly found in telephony is that of an electrical signal reflection due to imperfect impedence match at four-wire to two-wire conversion points commonly called hybrids. Thus, part of a near end party's speech signals are coupled from his speakerphone microphone circuit to his speaker circuit. A long delay introduced into this coupled portion of speech may cause confusion to the speaker and disruption of communications. The second form of echo unique to speakerphones is that of acoustic coupling between the speaker and the microphone. Acoustic energy produced by the speaker may reflect from nearby objects or be coupled directly to the microphone. This feedback produces a phase delay in the coupled audio signal making a hollow sounding echo and, with sufficient audio signal amplification gain, singing in the telephone circuit.
The conventional solution to echo and singing problems for speakerphones is the attenuation of the audio signal in the path having the weakest audio signal in favor of the path having the strongest audio signal. Many techniques have been developed to switch or variably attenuate one path or the other. These techniques, however, suffer from a number of problems including inability to accomodate high ambient noise environments, clipping of speech syllables of the party attempting to start a conversation on the attenuated path, and path attenuation uncertainty due to feedback from the speaker to the microphone when the speaker amplification has been turned up. These problems have been reduced by inventions (U.S. Pat. No. 4,378,603 by Eastmond and U.S. Pat. No. 4,400,584 by Vilmur) having unique speech detector circuits and special control processes.
For true duplex systems, self-adapting echo cancellers have recently been employed to mitigate echos by generating an estimate of the echo and subtracting the estimate from the signal corrupted by the echo. In this way, the acoustic coupling of a far end telephone talker can be removed from the signal generated by the near end speakerphone microphone without decreasing or switching the gain available to the near end speakerphone user. In a similar manner, electronic reflections from the hybrid may be cancelled at the near end speakerphone.
The determination of the echo estimate, however, may not always be calculated properly. Under conditions of high ambient noise, non-linear distortion, or both parties talking simultaneously, the echo sample may not be fully represenative of the echo signal. Therefore, there is a need for full duplex speakerphone which can provide optimum performance under less than ideal situations of noise, echo signal distortion, and simultaneous user speech.